The average bathroom fan moves about 200 cubic feet of air per minute. Cooker hood fans range from 150 cfm to 300 cfm and clothes dryer fans have higher capacities. The volume of air in a house with 8 ft. ceilings and 2,000 square feet of area is 16,000. cubic feet. A 200 cfm fan would change the air in 80 minutes. If several fans were used the house could receive several complete air changes by running the fans for 1 hour after dawn, before the sun has caused any significant temperature rise.
Photo electric devices fall into two classes. Photo transistors which need a power source and Photo cells which generate electric current when light falls upon them. Either can be used but use of the Photocell type may simplify wiring problems if a fan switch is on the live wire and the neutral is not adjacent. It is well within the known art to use a photo electric device to switch a current on or off. Timers are available which can have a variable response and which can be designed to operate a switch after a variable preset time. These devices are very well known and are not part of the invention except as means of carrying out the method of the invention.